Drive mechanisms for converting reciprocating or oscillating motion to intermittent rotary motion for use in counters, stepping switches or the like have been known heretofore.
One such drive mechanism has used a pawl, pivotally mounted, on a separately pivotally mounted oscillating driving member for stepping a toothed ratchet wheel. Another such drive mechanism has used an escapement type mechanism with two fixed pawls on a pivotally mounted driving member.
Such escapement type mechanism is generally low in cost since it comprises a single piece drive member with the two fixed pawls as an integral part of the drive member. The drive member pivots on a separate shaft or stud and as the fixed pawls, engaging the star wheel ratchet during a counter drive cycle, rotate the star wheel one half of the numerical value rotation during the impulse movement and the other half during the return movement of the reciprocating stroke, for example, in a ten digit per revolution, 36 degree per drive cycle device, the ratchet wheel is rotated 18 degrees for each half of the drive cycle. As the pivotally mounted drive member is driven during the impulse half of the oscillation, the tip of one of the fixed pawls disengages a tooth of the ratchet wheel at one circumferential side thereof and after sufficient movement of the driving member to assure clearance of the first pawl tip and the corresponding tooth, as the ratchet wheel would rotate, has occurred, the second fixed pawl engages a tooth of the star wheel ratchet at the other circumferential side thereof, driving the ratchet a first increment of rotation. At the end of the impulse movement of the oscillation such second, fixed pawl tip, having fully engaged the star wheel ratchet tooth, holds the ratchet in a fixed position. During this impulse movement the first, fixed pawl has moved laterally to the ratchet center-line to assure clearance beyond the outer diameter of the star wheel ratchet. On the return movement of the oscillation, the second increment of the numerical value rotation occurs in the same fashion but with the opposite pawl driving the ratchet. At certain significantly large portions of the driving cycle, the star wheel is unrestrained by the pawls and, if moved by vibration or shock, the drive system can malfunction. The escapement system is an inefficient coupling mechanism due to the large pawl to ratchet wheel clearances required. There is much lost motion in the cooperation between the escapement and the star wheel and, due to the lateral movement of the fixed pawls to provide clearance while the ratchet rotates, a large proportion of the coupling involves sliding motion, friction and wear. To provide sufficient motion of the fixed pawl tips from a typically small available motion of electromechanical prime movers generally in use on counters and the like, a large ratio from prime mover to pawl tips must be provided. The dimensional constraints to achieve this ratio and the geometry requirements between the driving member pivot center-line, the ratchet wheel center-line, and the dimensional criteria for the star wheel teeth tips and the driving member fixed pawl tips and finally the higher wear and erosion of these relative dimensional criteria, due to the high impact caused by the driving member being able to attain a high velocity before engaging the star wheel, is cause for reduction in count life of the mechanism or the addition of manufacturing costs to overcome these effects.
Present pivotally or flexibly mounted pawl drive systems overcome many of these escapement drive difficulties but require more parts and are more costly. Typically the prime mover reciprocating motion is converted to an arcuate motion of a pivoted lever, one end of which contains a pivotally or flexibly, shaft-mounted, spring-loaded pawl. The drive mechanism generally rotates the ratchet wheel a full numerical value step of rotation, for example, 36 degrees for a ten digit counter system during either the impulse or return movement of the driving cycle while the opposite movement is used in cocking the pawl-lever member, that is, storing energy in a spring for driving the ratchet or returning the pawl-lever member to the original position for the next cycle. This mechanism must provide a means for stopping the rotation of the ratchet wheel at the end of the desired angular rotation, since the pivotally or flexibly mounted pawl would permit the ratchet wheel to continue to rotate, inhibited only by the spring force of the pawl spring holding the pawl against the ratchet tooth. With the high rotational velocity developed during the driving half of the cycle, used to rotate a complete step, the impact and frictional wear generated in stopping the ratchet wheel at the desired position can be considerable. Further, an anti-backup means must be provided to prevent reverse rotational movement of the ratchet wheel during the cocking movement of the pawl. This reverse rotation is caused by the sliding friction and pawl spring bias force of the pawl against the ratchet tooth as the pawl slides over and hooks the next ratchet tooth for the next cycle. These driving mechanisms must generally provide a greater stroke at the pawl than is provided by commonly used electromechanical prime movers and therefore require a ratio in the pivoted lever assembly which, combined with the carefully located geometry required between the driving member pivot center-line, the pawl pivot center-line or location of the pawl tip when flexibly mounted, and the ratchet wheel center-line, the stopping means dimensional requirements as related to the ratchet wheel teeth and the anti-backup means dimensional requirements as related to the ratchet wheel teeth, escalates manufacturing and assembly costs.
While these prior drive mechanisms have been useful for their intended purposes, they nevertheless have had certain disadvantages such as low efficiency, high cost, lack of design flexibility, noisy operation, short life, limited speed capability and poor reliability. Therefore, it has become desirable to provide an improved drive mechanism that overcomes such disadvantages, which drive mechanism may be used to convert rectilinear motion to intermittent rotary motion in a large variety of applications such as counters, timers, metering devices, positioning sensors, indicators and the like.